1. Field of the Invention
The present invention relates to an optical star coupler and, more particularly, to an M.times.N amplified star coupler.
2. Description of the Prior Art
Rare earth-doped fiber amplifiers are beginning to find a wide variety of uses in optical communication systems. In particular, such amplifiers have been suggested for use in multichannel distribution networks. An article entitled "16-Channel Optical FDM Distribution/Transmission Experiment Utilizing Er.sup.3+ -Doped Fibre Amplifier", by H. Toba et al. appearing in Electronics Letters, Vol. 25, No. 14, July 1989, discusses one such broadcast arrangement. In particular, Toba et al. describe a system where sixteen DFB lasers, operating at 5 GHz frequency intervals, are applied as inputs to a 16.times.16 star coupler. The multiplexed signals at each output port are then combined with a pump laser operating at 1.48 .mu.m and applied as an input to an isolated Er.sup.3+ -doped fiber amplifier. The amplified signals are then transmitted over single mode fiber to the predefined destination. The arrangement as described utilizes a discrete fiber amplifier arrangement (i.e., pump source, couplers, doped fiber) for each of the sixteen output fibers. For large systems, the number of additional components required to provide amplification may become cost and size prohibitive.
An alternative arrangement is disclosed in an article entitled "7203-User WDM Broadcast Network Employing One Erbium-Doped Fibre Power Amplifier", by A. M. Hill et al. appearing in Electronic Letters, Vol. 26, No. 9, April 1990 at pp. 605-7. The network in this case uses a series of ten DFB lasers to transmit 320 digitized video channels. The channels are multiplexed at a remote location using a number of separate 2.times.2 couplers to form a relatively large number of separate multiplexed signals. At an intermediate location, a particular multiplexed signal passes through an isolated erbium-doped fiber amplifier and subsequently through five stages of various passive couplers (including 1.times.7 and 3.times.3 fused fiber couplers) to provide signals to the 7203 users. The use of the multiply-cascaded couplers in this arrangement, however, is reported to introduce a loss of approximately 40 dB into the system. For most applications such a loss in unacceptable.
Thus, a need remains in the art for a viable communication alternative which exploits the benefits of fiber amplifiers, without incurring the limitations as discussed above.